The present invention relates to a method of separating oxygen from air, an oxygen exchange material useful therefor, and to a method for preparing said material. More particularly, the invention relates to praseodymium-cerium oxide materials which are useful in air separation processes and to a method of preparing said oxide materials.
The increasing industrial demand for pure oxygen has focused scientific attention on chemical techniques for separating oxygen from nitrogen and the other gases found in admixture in the atmosphere. Among the known physical methods of air separation, fractionation of liquefied air is by far the most widely used commercial process.
Chemical processes which have been investigated are, for the most part, predicated on the ability of certain chemical compounds or elements to undergo a reversible chemical reaction with oxygen such that they selectively combine with oxygen in the atmosphere under one set of operating conditions (i.e. temperature and pressure) and thereafter are decomposed under a different set of conditions to yield oxygen and the original reactant. Numerous methods of cycling the oxygen carrier material have been considered. It has been proposed, for example, that the chemical reaction mass be in the form of a fixed bed with the gaseous atmosphere in contact therewith being cycled by pressure and/or temperature swings. Alternatively, it has been suggested that the reaction mass be moved continuously through absorption and desorption zones using conventional fluidized bed techniques.
Although conceptually attractive, the use of reversible chemical reaction masses for air separation has not proven commercially feasible for reasons varying with the particular reaction mass studied. Three well-known processes -- the Mallet process, the Brin process and the duMotay process -- are reviewed in U.S. Pat. No. 3,579,292 directed to a chemical air separation process characterized by the reversible reaction of strontium oxide with oxygen to form strontium peroxide.
In general, there are thee principal requirements to be met before a chemical process for manufacturing oxygen can be considered practical from a commercial standpoint. These are: (1) a reaction mass having a reversible chemical equilibrium with oxygen at reasonably low operating temperatures and at convenient pressures, e.g., oxygen equilibrium pressures greater than about 0.2 atm. below about 500.degree.C, (2) usable oxygen loadings and fast intrinsic kinetics so that relatively rapid reaction rates are attainable at practical driving forces, and (3) chemical and physical stability such that: (a) competing side reactions are absent, (b) reaction rates are invariant with time, especially during repeated cycling, (c) the reaction mass is relatively inert to air contaminants such as CO.sub.2 and H.sub.2 O, and (d) the material can be handled and used without undue losses from attrition, sintering or vaporization, for example.
Praseodymium oxides have been suggested in the art as oxygen exchange materials for an air separation process. In an article in the Journal of Chemical Education (Vol. 40, 1963, p. 150) P. A. Faeth and A. F. Clifford, proposed the use of a Pr-O system for generating oxygen to take advantage of the relative ease with which oxygen can be removed from and combined with oxides of praseodymium. Unfortunately, however, the operating conditions which are required for generating oxygen are so impractical as to preclude any serious interest in praseodymium oxides as commercially feasible chemical reaction materials. For example, the equilibrium pressure of O.sub.2 is stated to be no greater than 160 mm at temperatures below 900.degree.C. Thus, if the reaction were carried out on a commercial scale, the extremely high operating temperatures would necessitate relatively expensive heat resistant reactors and flow equipment. Furthermore, a maximum oxygen pressure of 160 mm would necessitate vacuum pumping during the dissociation cycle followed by an inordinate amount of gas compression in order to deliver the generated oxygen at a practical operating pressure. Moreover, the temperature swing of about 450.degree.C which was effected to cycle the praseodymium oxide reactant from an oxidized phase to a reduced phase would be economically prohibitive in terms of continuously heating and cooling a large mass of material over a temperature range of several hundred degrees.